Two-Phase Olive Mill Waste: A Circular Economy Solution to an Imminent Problem in Southern Europe

Abstract The present study proposes the possibility of implementing a clean, renewable technology that allow profitable value to be drawn from a highly contaminant waste, Two Phase Olive Mill Waste (TPOMW). An anaerobic digestion (AD) process under optimal conditions can significantly reduce the overall chemical oxygen demand (COD) by 46.87± 4.53% and ensure the stability of the process. For this the TPOMW has to be subjected to three procedures: (i) dilution, mixture formed by 30% de TPOMW and 70% water; (ii) a 5-day aeration pre-treatment at a 0.65 L/minute flow rate of air per L of reactor; and (iii) pH monitoring and adjustment to 7 by the addition of a 10% calcium hydroxide solution. After these actions, biomethanation of TPOMW yielding a mean production of 12.69 ± 1.57 Nm³ methane/m³ substrate, allowing the sale of excess thermal energy to nearby companies. The study of the economic feasibility of an industrial plant, based on previous results, showed that it isn´t profitable economically because the technology is emerging and it is necessary to improve process performance and reduce installation, operation and maintenance costs. However, the project presented is environmentally viable, since the carbon dioxide emissions that would be released to the atmosphere if the TPOMW were to decompose naturally are reduced by a factor of 7 times. This study thus constitutes an example of a "green and circular economy".